Production of metal castings



United States Patent 3,473,509 PRODUCTION OF METAL CASTllJGS Kenneth Rose, Lichfield, England, assignor to Doulton & Co. Limited, London, England, a company of Great Britain No Drawing. Filed Jan. 13, 1966, Ser. No. 520,397 Claims priority, application Great Britain, Jan. 19, 1965, 2,306/65 Int. Cl. B22: 1/14; C01f 11/00 US. Cl. 164-41 9 Claims ABSTRACT OF THE DISCLOSURE This invention relates to the production of metal castings, including in particular the production of aluminium alloy castings.

According to the present invention, a casting of an aluminium alloy or another metal or alloy inert to nitric acid is produced by a method comprising forming a mould assembly of a mould and/or c0re(s) constituted by a coherent calcium phosphate composition, which may be a coherent calcined calcium phosphate composition, introducing into the mould assembly a charge of the metal or alloy in question in molten form, allowing the charge to solidify, and removing the mould and/or core(s) from the casting so formed. This removal may be at least in part effected by dissolving in nitric acid. The words calcium phosphate are used in this specification to mean tricalcium diorthophosphate, Ca (PO,,) with or without other calcium phosphate materials having calcium and phosphorus contents of 2545 and 12-30% by weight respectively, and the words calcium phosphate composition are to be understood to denote any composition thereof which upon being treated with nitric acid will either dissolve altogether or dissolve leaving anly discrete particles.

The invention also includes castings produced by a method as specified in the preceding paragraph, and further includes, as products novel in themselves, certain moulds and mould-cores constituted by a coherent calcium phosphate composition.

Investment casting, die casting, and sand casting may all be carried out in accordance with the present invention. In all three, preformed cores of the calcium phosphate composition may be used, and in investment casting the moulds also may be of the calcium phosphate composition; furthermore, when in investment casting, calcium phosphate composition cores are used, these cores may be invested-in cores, i.e., cores that have been formed in the Wax pattern. The calcium phosphate composition may in addition he used for pre-formed expendable moulds as well as for the in situ-formed expendable moulds for the investment-casting technique. Naturally, however, the value of such moulds is to be found more particularly in the field of castings having complicated configurations with re-entrant features.

The proportion by weight of calcium phosphate itself in the calcium phosphate composition used in accordance With the present invention may vary from 100% down to as low a value as 20%. Up to 80% by weight of the composition may thus be represented by ingredients other 3,473,599 Patented Oct. 21, 1969 ice than calcium phosphate, and these include in particular, apart from impurities or contaminants, other more or less refractory substances, examples of which are magnesium oxide, alumina, zircon (ZrSi silica, zirconia (ZrO mullite, molochite, and sillimanite. Various other substances useful in the preparation of the moulds and cores as temporary binders may also be introduced, before the firing step, and some examples of these are paraffin wax dissolved for mixing purposes in trichloroethylene, aqueous ammonium alginate, and binders based on ethyl silicate. It is also possible to use binders that on airdrying or baking give moulds and cores strong enough to be used in an unfired state; these include sodium silicate and aluminium orthophosphate solutions, and resin binders.

The calcium phosphate may be chemically precipitated phosphate and/ or bone ash phosphate and/ or rock phosphate (i.e., naturally occurring calcium phosphate). In the former case, however, the particle size should for the present purpose preferably be increased, or alternatively the precipitated phosphate should be augmented by other ingredients which will facilitate mould and core fabrication and reduce shrinkage on firing of the mould or core. Bone ash and rock phosphate, on the other hand, are available in particle sizes suitable for the present purpose and may accordingly be used in accordance with the present invention without modification (other than compounding as desired). The bone ash materials which may be used include those which contain hydroxy-apatite, I..,

The product obtained by calcining dicalcium phosphate, i.e., CaHPO -2H O- at 800-1300 C., may also be used.

As already implied, the preparation of the moulds or cores of the present invention will include a firing step unless an air-drying or baking binder giving a strong unfired core or mould is used. The temperature reached in this firing step may suitably be as high as 1000 to 1400 C., but it should be appreciated that if the moulds or cores are prepared from compositions incorporating temporary binders a preliminary heating step at a lower temperature (e.g., in the range to 400 C.) may be carried out, in which the temporary binder is driven off or decomposed, according to its constitution. Furthermore, in the case of a chemically precipitated calcium phosphate, a still earlier heating step may be included if desired, this being a calcination step carried out before the precipitated phosphate is mixed with the other ingredients (if any); normally such a preliminary calcination step should be followed by comminution of the calcination product to the particle sizes required.

The method by which the mould or core is shaped will naturally depend upon the configuration required. However, the methods of shaping which may be used include extrusion, injection moulding, compression moulding, ramming, isostatic pressing, and slip casting, and also core blowing, tamping and chemical casting.

In the case of cores made from materials bonded with sodium silicate, the sodium silicate can be removed by hot or cold water or steam, leaving a loose mass of calcium phosphate that can either be shaken out or otherwise mechanically removed from the casting, or be removed chemically by subsequent immersion in nitric acid.

The following examples illustrate the invention.

EXAMPLE I 1 kilogram of a chemically precipitated calcium phosphate powder with an average particle size (as determined by an air-permeability technique) of 0.5 mircon was loaded into an alumina container, heated to 1250 C. and kept at that temperature for 2 hours. After cooling, the calcine produced was comminuted to pass a 200- Transverse breaking strength (lb./sq. inch) 1,100 Bulk density (gms./cc.) 1.78 Firing shrinkage (percent) 2 Pieces of approximate size 2 x 1 x 0.1 inch obtained by cutting the test bars were found to dissolve in a 50% solution of nitric acid (specific gravity=1.42) in water inhour. Similarly sized pieces were encased with aluminium by pouring molten commercial-purity aluminium (at 820 C.) around them. No chemical reactions were observed. After cooling, the ceramic was readily removed by 50% nitric acid in 1 hour.

EXAMPLE II A comminuted calcine, with an average particle size of 4.1 microns, produced as in Example I, was dry ballmilled for 1 hour. Average particle size was again measured and found to be 3.5 microns. 70 grams of this material was dry-mixed with 30 grams of a tabular alumina material that passed a 100 ES. sieve but was retained by a 200 ES. sieve. 4% by weight of paraifin wax was added as a solution in trichlorethylene, the latter then being removed by drying at 120 C. The resulting cake was rubbed through a 20 BS. sieve and pressed at 4 tons per sq. inch in a steel die of size 2 x 1 inch to produce test bars of size 2 x 1 x 0.13 inch. The test bars so formed were fired to 1190 C. (being raised to 1190 C. at a rate of 43 C. per hour) and kept at 1190 C. for /2 hour before cooling. The fired test bars had the following properties:

Transverse breaking strength (lb./ sq. inch) 500 Bulk density (g./cc.) 1.89 Firing shrinkage (percent) 0.4

As in Example I, the test bars dissolved in 50% nitric acid in 1 hour both when unencased and after having aluminium cast around them.

EXAMPLE III 400 grams of a chemically precipitated calcium phosphate powder with an average particle size of 0.5 micron was dry-mixed for 1 hour in a Y-cone blender with 600 grams of a calcined magnesite of average particle size of 1.1 microns. 4% by weight of paraffin wax was added and 2 x 1 x 0.13 inch test bars were formed and fired as in Example II. The fired test bars had the following properties:

Transverse breaking strength (lb./ sq. inch) 600 Bulk density (gm/cc.) 1.62 Firing shrinkage (percent) 4 As in Example I, the test bars dissolved in 50% nitric acid in 1 hour both when unencased and after having aluminium cast around them.

EXAMPLE IV 200 grams of a chemically precipitated calcium phosphate with an average particle size of 0.5 micron were dry-mixed with 800 grams of a bone ash sample of average particle size 11.3 microns. The mixture was plasticised with water and a little ammonium alginate, and extruded through a die of inside diameter 0.5 inch to form rods approximately inches long. These rods were airdried and fired to 1250 C. (being raised to 1250 C. at a rate of 70 C. per hour), kept at 1250 C. for 2 hours 4 and then cooled. The fired rods had the following properties:

Transverse breaking strength (lb./sq. inch) 1,200 Bulk density (gm/cc.) 1.98 Firing shrinkage (percent) [.6

2-inch lengths of the 10-inch rods were used as preformed cores in shell castings produced from a conventional aluminium casting alloy. After casting, the cores were removed from the castings with 50% nitric acid. the time taken being 2 hours.

EXAMPLE V A mixture of powders was prepared as in Example IV. 382 grams of this powder was added to a mixture of 98 ml. of ethyl silicate, 2 ml. of piperidine, 17 ml. of isopropyl alcohol and 3 ml. of distilled water. The paste so produced was well mixed and then rammed into a cavity in a wax pattern used for investment casting. A ceramic shell was then built up around this pattern using orthodox investment casting methods. The wax was eliminated from the shell and rammed core at 1300 C. and a commercial aluminium alloy was poured into the shell and so around the now densified core. After cooling, the shell was removed and the core dissolved in 50% nitric acid.

EXAMPLE VI 1 kg. of a calcined bone material that had passed a 44 BS. sieve was mixed for ten minutes with 200 grams of a solution of sodium silicate in water that contained 9% Na O, 30% SiO and 61% H O, by Weight. The resulting damp mix was placed in a commercial core-blower and was blown into a mould with a cylindrical cavity of length 8 ins. and diameter 0.5 in. On the completion of blowing, carbon dioxide gas was passed through the mould for one minute to harden the formed shape. The mould was stripped and the cylindrical bar inside was removed. The procedure was repeated to make six bars, and the bars formed were baked overnight at C. The cross breaking strengths of two bars were determined and found to be 40 lb./in. A further bar was used as a core in an experimental gravity die casting mould into which a molten aluminium alloy was then poured. The casting produced was placed in an agitated bath of boiling water for 30 mins. to remove most of the sodium silicate. After this treatment much of the core could be removed by shaking. The casting was placed in a bath of nitric acid with a specific gravity of 1.42 for 5 mins. to remove the small traces of calcium phosphate material still remaining.

The remaining three bars were vacuum-infiltrated with a solution of sodium silicate in water that contained 15% Na O, 30% SiO and 55% H O, by Weight. The infiltrated bars were placed on a fine grid to drain, and were then exposed to a stream of carbon dioxide gas for 10 mins. They were subsequently baked overnight to 100 C. The cross breaking strengths of two bars were determined and were found to average 3,200 lb./in. The third bar was used as a core in an experimental pressure die casting mould and a molten aluminium alloy was injected at 10,000 1b./in. The solidified casting was removed and soaked, firstly in boiling water for 1 hr. and then in 50% nitric acid for 20 mins. to remove the core.

EXAMPLE VII 800 grams of a calcined bone ash material that had passed a 44 BS. sieve was dry mixed for 10 mins. with 200 grams of a chemically precipitated calcium phosphate with an average particle size of 0.5 micron. 180 grams of a solution of aluminium orthophosphate in water containing about 40% dissolved alumina and phosphoric oxide was then added and mixed in.

The resulting dampened powder was rubbed through a 16 BS. sieve and pressed in a steel tool at 1.5 ton/in.

to form test bars 4 ins. long x 1 in. wide x 0.15 in. thick. The test bars were baked at 180 C. overnight. Cross breaking strengths were then determined and were found to average 600 lb./in. The half bars left after the strength determinations were completely dissolved in 50% nitric acid in 1 hr. when uncased and after having aluminium cast around them in an experimental gravity die casting mould.

I claim:

1. A method of producing a casting of metallic material inert to nitric acid, said method comprising forming a mould assembly including a mould structural component comprising a coherent mass of powder size particles of calcium phosphate composition comprising tricalcium diorthophosphate in the amount of more than 20% by weight of said mould component, said calcium phosphate composition having a calcium content of 2545% by weight and a phosphorous content of 1230% by weight and being soluble in nitric acid of substantially 50% strength, introducing into the mould assembly a charge of the metallic material in molten form, allowing the charge to solidify, and removing said mass of calcium phosphate composition from the casting by dissolving the composition in nitric acid of substantially 50% strength.

2. A method according to claim 1 in which the coherent calcium phosphate composition used is a coherent calcined calcium phosphate composition.

3. A method according to claim 1 in which said mould component contains up to 80% by weight of other refractory material.

4. A method according to claim 3, in which the other refractory material used comprises material selected from the group consisting of magnesium oxide, alumina, zircon, silica, Zirconia, mullite, molochite and sillimanite.

5. A method according to claim 1 in which said mould component is prepared with the use of a temporary binder.

6. A method according to claim 1 in which the calcium phosphate composition used is prepared with the use of a binder of the class consisting of air-drying and baking binders.

7. A method according to claim 1, in which a sodium silicate solution binder is incorporated in said mould component with said calcium phosphate composition, and in which said sodium silicate binder is dissolved out of said mould component with water after said metallic material has solidified and before said calcium phosphate composition has been dissolved in the nitric acid.

8. A method according to claim 1 in which heat is applied to the particulate calcium phosphate composition to effect coherence of said particles.

9. A method according to claim 1 in which the particulate calcium phosphate composition is wetted to form a paste and is then dried to etlect coherence of said particles.

References Cited UNITED STATES PATENTS 1,544,929 7/1925 Pack 164-132 XR 2,181,004 11/1939 Salzberg 164-41 XR 2,522,548 9/1950 Streicher 164-41 2,086,653 7/1937 Watson et a1. 164-132 XR 2,368,296 1/1945 Goran 164-132 XR 2,680,890 6/1954 Moore et a1 164-41 3,356,129 12/1967 Anderko et a1. 164-36 X 2,479,504 8/1949 Moore et al. 106-38 2,895,838 7/1959 Ilenda 106-38 2,879,169 3/ 1959 Teicher 106-38.27 2,618,530 11/ 1952 Gardner.

2,116,207 5/1938 Lindner et al. 106-38.27 X 2,023,957 12/1935 Hewgill 106-38 J. SPENCER OVERHOLSER, Primary Examiner V. RISING, Assistant Examiner US. Cl, X.R. 

